1. Field of the Invention
The invention relates to a spindle motor for rotatably driving a magnetic disc, an optical disc and the like, and a hydrodynamic bearing device used in the spindle motor and the like.
2. Description of the Related Art
Instead of a ball bearing device conventionally used, a hydrodynamic bearing device excelling in rotating precision and silence than the ball bearing is widely adopted for the bearing device used in the spindle motor and the like of a hard disc drive.
This type of hydrodynamic bearing device includes a hydrodynamic bearing device disclosed in for example, JP-A 11-82486 (1999). As shown in FIG. 13, the hydrodynamic bearing device includes a shaft 51, a sleeve 52 arranged on the outer periphery of the shaft 51 with a gap in between, and thrust flanges 53 and 54 of thick diameter arranged on both ends of the shaft 51 and arranged in an orientation that includes a gap with respect to both end faces of the sleeve 52. A working fluid consisting of lubricant oil is filled into the gap between the outer peripheral surface of the shaft 51 and the inner peripheral surface of the sleeve 52, and the gap between the surface on the inner side of the thrust flanges 53 and 54 (lower surface of the thrust flange 53 and the upper surface of the thrust flange 54) and both end faces of the sleeve 52 facing the respective surface of the flange. A dynamic pressure generating groove 56 is formed on the outer peripheral surface of the shaft 51 and a radial hydrodynamic bearing is configured in which, when the shaft 51 and the sleeve 52 are relatively rotated by the motor rotational driving force not shown, the shaft 51 and the sleeve 52 are supported in a freely rotating manner through a predetermined gap in a radial direction (direction of radius) by the pressure of the working fluid collected by the dynamic pressure generating groove 56. The dynamic pressure generating grooves 57 and 58 are formed on the surface on the inner side of the of the thrust flanges 53 and 54 and a thrust hydrodynamic bearing is configured in which, when the thrust flanges 53 and 54 attached to the shaft 51 and the sleeve 52 are relatively rotated by the motor rotational driving force, the shaft 51 and the sleeve 52 are supported in a freely rotating manner through a predetermined gap in a thrust direction (direction of bearing axis) by the pressure of the working fluid collected by the dynamic pressure generating grooves 57 and 58.
In this hydrodynamic bearing device, a plurality of communication paths 59 extending parallel to the bearing axis are formed at an intermediate location between the inner peripheral surface and the outer peripheral surface of the sleeve 52 at every appropriate angle (e.g., 180°) with the bearing axis as the center. A space between the surface on the inner side of the thrust flanges 53 and 54 and both end faces of the sleeve 52 facing the respective surface of the flange is communicated by the communication paths 59. Fluid closing members 60 and 61 are fitted to the inner peripheral part of both ends of the sleeve 52 so as to face the outer peripheral surface of the thrust flanges 53 and 54 across a clearance. Inclined surfaces 60a and 61a of conical shape are formed at the location of the fluid closing members 60 and 61 facing the communication paths 59, and the locations facing the inclined surfaces 60a and 61a become the fluid storage spaces 64 and 65 where the working fluid is stored. The clearance is formed between the outer peripheral surface of the thrust flanges 53 and 54 and the inner peripheral surfaces of the fluid closing members 60 and 61 and is communicated to the outside air (atmospheric pressure). Fluid sealing parts 62 and 63 for sealing the working fluid on the internal side of the hydrodynamic bearing device are also arranged using the surface tension of the working fluid.
Therefore, the configuration given above is such in which even when the pressure of the working fluid becomes uneven at the space between the outer peripheral surface of the shaft 51 formed with the radial hydrodynamic bearing and the inner peripheral surface of the sleeve 52, and the space between the surface on the inner side of the thrust flanges 53 and 54 formed with the thrust hydrodynamic bearing and both end faces of the sleeve 52 facing the respective surface of the flange due to formation of the communication paths 59, and a pressure difference is created, such pressure difference is resolved. That is, even if the pressure of the working fluid becomes uneven by arranging the communication paths 59, adjustment is made to eliminate the pressure difference between the working fluids to stabilize the bearing function or to prevent the working fluid from jumping outward.
In the general hydrodynamic bearing device of this type, the clearance where the radial hydrodynamic bearing is formed or the clearance where the thrust hydrodynamic bearing is formed is extremely small, and thus the work of assembling the hydrodynamic bearing device and then filling the working fluid into the hydrodynamic bearing includes filling the working fluid to the inner part of the hydrodynamic bearing device so that the fluid is satisfactorily filled to the inner part. Even so, however, one part of air sometimes enters or remains in the space between the outer peripheral surface of the shaft 51 formed with the radial hydrodynamic bearing and the inner peripheral surface of the sleeve 52, and the space between the surface on the inner side of the thrust flanges 53 and 54 formed with the thrust hydrodynamic bearing and both end faces of the sleeve 52 facing the respective surface of the flange. Further, rotation of the hydrodynamic bearing device may involve and mix small air bubbles in the working fluid. Therefore, when the air enters the inner part as air bubbles and attaches to the dynamic pressure generating groove 56 of the radial hydrodynamic bearing or the dynamic pressure generating grooves 57 and 58 of the thrust hydrodynamic bearing, the pressure generated by the working fluid in the dynamic pressure generating grooves 56, 57 and 58 becomes disturbed, thus lowering the bearing performance such as, lower bearing stiffness due to air bubbles, and instability of rotation during rotating operation.
The fluid closing members for sealing the working fluid in the bearing are fixed to the sleeve, but if adhesive and the like are used to enhance sealability, the adhesive may flow into the bearing thereby causing disadvantages such as lock.